This invention relates to alkaline galvanic cells having zinc electrodes and an alkaline electrolyte. More particularly, the present invention relates to high energy density rechargeable cells having a zinc or zinc-based negative electrodes, an alkaline electrolyte, and positive electrodes which may be nickel, silver, air, or iron.
The provision of rechargeable zinc batteries having alkaline electrolytes is well known. Leaving aside the question of zinc/manganese dioxide cells, which find dominance in commercial fields supplying cells (batteries) for use in flashlights, toys, low drainage devices such as electric clocks, and the like, there is also a very large market and requirement for high energy density, high capacity cells and batteries such as nickel-zinc, silver-zinc, and zinc-air batteries, as well as a recently introduced super iron-zinc battery. A requirement is, however, that such cells and batteries must be cycled many times through discharge/charge cycles; leading in turn to several further requirements. The first is that the capacity of the rechargeable cell should not diminish significantly over a number of cycles, there should be no significant shape changexe2x80x94particularly of the zinc electrodexe2x80x94and no significant dendrite formation. Most especially, newly developed high energy density rechargeable zinc cells should be free or substantially free of toxicity, so as to be environmentally benign. This means, in particularly, that a robust, long-lasting, rechargeable battery must be brought to the market which contains no cadmium, no lead, and no mercury.
In the following discussion, the terms xe2x80x9ccellxe2x80x9d and xe2x80x9cbatteryxe2x80x9d may be used interchangeably. Of course, it is recognized that a cell comprises two electrodes, one positive and one negative, and an electrolyte; and a battery may comprise a number of cells which are joined together in series, parallel, or series/parallel. In many batteries, of course, there are a plurality of negative and positive plates and a common electrolyte all contained in a single casing; and in some cases, the plates may be bipolar. In other batteries, there may be a plurality of self-contained cells, each having their own positive and negative electrodes and electrolyte. Moreover, cells and batteries may be cylindrical or rectangular, they may comprise flat plates or rolled plates, and they may have a relatively low voltage of one or two volts. Batteries may have a relatively high voltage, in the range of twelve to sixteen volts, sometimes much higher. The present invention applies to any of the above considerations in respect of cells and batteries, as it relates to the structure of the electrodes.
Batteries in keeping with the present invention will find particular utility in traction markets, being used on scooters and bicycles, as well as in hybrid automobiles, and in the power tool market. In such markets, there is a very great necessity for sealed battery systems, which have high energy density and, therefore, relatively low weight per amp-hour unit capacity, as well as low cost per amp-hour unit capacity. Moreover, batteries in keeping with the present invention will provide high amp-hour capacity per unit volume.
A particular feature of the present invention is that long life, rechargeable zinc electrodes may be supplied which can be manufactured using equipment that is presently used for nickel cadmium or nickel metal-hydride production. Typically, electrodes which embody the present invention are pasted on a high conductivity, low resistance matrix or carrier, which might conveniently be manufactured from copper and/or suitable alloys thereof.
In order to minimize shape change and dendrite growth, and to provide a stable construction so as to achieve prolonged cycle life, high power over the life of the battery, and maximum energy density, careful choice must be made of the combination of the zinc electrode and the electrolyte for the battery.
Typically, batteries in keeping with the present invention employ a buffered borate electrolyte which is particularly such as that which is taught in EISENBERG U.S. Pat. No. 5,235,836, issued Jun. 1, 1993. That patent teaches alkaline galvanic cells having an electrolyte composition which comprises a solution of a salt formed by reacting boric acid (or, alternatively, phosphoric acid or arsenic acid) with an alkali or earth alkali hydroxide which is present in a sufficient amount to produce a stoichiometric excess of hydroxide to acid in the range of 2.5 to 11.0 equivalents per liter. As well, a soluble alkali or earth or earth alkali fluoride is present, in an amount corresponding to a concentration range of 0.01 to 1.0 equivalents per liter of total solution.
By suitable zinc formulation, as taught hereafter, together with use of the buffered borate electrolyte, degradation mechanisms that operate during cycling are inhibited.
The performance of rechargeable zinc electrodes in alkaline electrolytes has been the subject of many studies that encompass the zinc electrode composition and the interaction with the electrolyte. A performance inhibiting disfigurement of the zinc electrode occurs as cycling progresses. The most obvious effect is a lower than acceptable amp-hour capacity delivered at useable voltages. This tendency has been arrested by a number of approaches. The combination of more dilute potassium hydroxide electrolyte together with the addition of calcium hydroxide to the zinc electrode appears to be particularly effective (U.S. Pat. No. 4,358,517 issued Nov. 9, 1982 to JONES). Alternate approaches have used buffered electrolytes with and without fluoride additions that have also resulted in increased zinc electrode life span. Noteworthy among these approaches is that described in U.S. Pat. No. 5,453,336 issued Sep. 26, 1995 to ADLER et al. which teaches a mixture of alkaline electrolyte (2-12 M) combined with a carbonate of 0.5-4 M and a fluoride of 0.5-4 M is particularly effective. U.S. Pat. No. 4,273,841 issued Jun. 16, 1981 to CARLSON describes another mixture that employs 5-10% hydroxide, 10-20% phosphate and 5-15% fluoride. EISENBERG describes two electrolyte formulations in U.S. Pat. No. 4,224,391 issued Sep. 23, 1980 and No. 5,215,836 issued Jun. 1, 1993. Both employ mixtures of potassium hydroxide and boric, phosphoric or arsenic acid. However the latter patent describes advantages of alkali fluorides in the range of 0.01 to 1 M. This should be implemented in combination with a more alkaline electrolyte.
Electrode additions of numerous oxide additives have been investigated and proved to be beneficial (U.S. Pat. No. 5,556,720 issued Sep. 17, 1996 to CHARKEY and No. 4,084,047 issued Apr. 11, 1978 to HIMY et al.). Some oxides are claimed to result in a conductive metal matrix that enhances electrode conductivity; others may result in the formation of beneficial alloys and a reduction in hydrogen evolution. Zinc fluoride and zinc titanate (U.S. Pat. No. 4,304,828 issued Dec. 8, 1981 to VAIDYANATHAN) and the direct additions of alkali fluoride to the zinc electrode have also been found beneficial (U.S. Pat. No. 4,017,665 issued Apr. 12, 1977 to SANDERA et al.).
It should also be noted that lead, cadmium, and mercury oxides are particularly favoured as electrode enhancement additives. However, these oxides impart a toxicity to the electrode that is undesirablexe2x80x94and, in some instances, unacceptable.